A machining center is a numerical control (NC) machining tool with an automatic tool-changing function. The machining center can automatically perform various working such as milling, drilling or notching, boring, tapping, etc. on works set thereto with improved easiness and efficiency.
To exchange and use a plurality of cutting tools in a desired manner, each tool is mounted to a tool holder and set on a tool stand. An example of the conventional tool holders is shown in FIGS. 22 and 23. This tool holder 1 comprises a tapered portion 2 adapted to be connected to a spindle of a machining center, a manipulator-engaging portion 3 and a chuck portion 4 for firmly holding the tool. A shank of the tool inserted into an aperture of the chuck portion 4 is fixed by contracting a needle roller 43 by nuts 41.
Present as a tool holder for filly holding a tool in addition to the above nut-type tool holder is a so-called shrinkage-fit tool holder. The shrinkage-fit tool holder has recently attracted much attention, because it can firmly hold a tool with an excellent dynamic balancing suitable for high-speed working. Shown in FIGS. 24(a) and (b) is an example of the shrinkage-fit tool holders. The tool holder 1 comprises a tapered connecting portion 2, a manipulator-engaging portion 3 and a chuck portion 4 for firmly holding a tool. A shank of the tool 9 is inserted into an aperture of the chuck portion 4 and secured by shrinkage fitting. A tool-holding portion formed in a front portion of the chuck portion 4 has an inner surface on which four grooves 45 for spraying a machining fluid or air are formed.
Shown in FIGS. 25(a) and (b) is another example of the shrinkage-fit tool holders. The tool holder 1 comprises a tapered connecting portion 2, a manipulator-engaging portion 3 and a chuck portion 4 for firmly holding a tool. A shank of the tool 9 inserted into an aperture of the chuck portion 4 is secured by shrinkage fitting. Four apertures 46 communicating with an aperture of the tool holder are formed in a tool-holding portion in a front portion of the chuck portion 4 in parallel with the axis of the tool holder, to spray a machining fluid or air from a tip end of the chuck portion 4.
The conventional shrinkage-fit tool holders utilize differences in thermal expansion coefficients between the tool-holding portions and the tool shanks. The tool shanks are made of materials having low thermal expansion coefficients such as cemented carbides, cermets, ceramics, invar, etc., while the tool-holding portions are made of high-expansion materials such as nickel-chromium steel, etc. (see, for instance, Japanese Utility Model Laid-Open No. 4-54606). Interference determining connecting strength (gripping strength) is approximately 1/1000 of a diameter of the tool shank. For example, it is approximately 0.02 mm in the case of the tool shank having a diameter of 20 to 30 mm. Meeting these conditions enables a heating temperature required for shrinkage fitting to be lowered.
In these prior art tool holders, the tool shank and the aperture of the tool-holding portion have suitably controlled sizes to achieve detachability, thereby minimizing heating for shrinkage fitting and thus preventing decrease in strength and hardness of materials due to change in their structures. Proposed in addition to the above tool holders is, for example, a shrinkage-fit tool holder comprising a member made of high-speed steel, stainless steel, etc. for firmly holding a cemented carbide tool shank by caulking, from which the tool shank is detached by heating (see, for instance, Japanese Utility Model Laid-Open No. 1-92309).
However, in the conventional shrinkage-fit tool holders, the difference in a thermal expansion coefficient between the tool-holding member and the tool shank is insufficient, thereby failing to achieve a sufficient gripping strength. Thermal expansion decreases in proportion to the reduction in a diameter of the tool shank. Thus, when the diameter of the tool shank is 12 mm or less, the thermal expansion is too small to obtain a sufficient gripping strength even with a small tolerance. Accordingly, almost all commercially available tools (mostly having shank diameters of 12 mm or less) cannot be subjected to shrinkage fitting.
Any tool shanks of JIS have tolerance, which is, for instance, h7 for solid cemented carbide end mills of JIS B4116. It is difficult to achieve a sufficient different in a thermal expansion coefficient to absorb this tolerance and thus a sufficient shrinkage fitting strength, with the conventional shrinkage-fit tool holders. To completely absorb the tolerance of the tool, the tool holder should have as small a tolerance as possible in an aperture of a holding member. This is, however, difficult particularly in the case of small-diameter tools. Though it may be considered to work an aperture of a tool-holding member in each tool holder such that it is adapted to a tool shank in-situ combination thereof, it is impossible from the viewpoint of mass productivity.
It may also be considered to elevate a shrinkage-fitting temperature to absorb such tolerance, and heating is now conducted up to about 700.degree. C. However, decrease in strength takes place in tool holders made of conventional materials, resulting in widening of the aperture of the holding member and deterioration in structures like tempering of steel by repeated heating and cooling, and thus decrease in hardness and strength of the tool holder. Further, it is likely that an oxide layer is formed in the aperture of the holding member, resulting in decrease in gripping strength and change in an inner diameter.
With respect to the structure of the tool holder, the entire tool holder is heated and cooled when a tool is directly shrinkage-fit into the tool holder. Thus, it takes too long time for shrinkage fitting, thereby making it difficult to achieve easy handling of the tool holder.
With respect to apertures for supplying a machining fluid or air, grooves are formed in an inner surface of a tool-holding member or apertures communicating to an end surface are formed as shown in FIGS. 24 and 25. However, because an extremely large stress is applied to a holding portion of the tool holder in a state of shrinkage-fitting the tool shank, the concentration of stress occurs in an aperture shape as shown in FIGS. 24 and 25. There is also a problem that the machining fluid or air is scattered by rotation of the tool, whereby it cannot be concentrated at a tip end of the tool.
Accordingly, an object of the present invention is to provide a shrinkage-fit tool holder solving problems inherent in the conventional tool holders, which shows sufficient gripping strength and can be flexibly used with commercially available tools.